Vehicles such as loaders are used generally to transfer bulk material from a stock pile onto transport vehicles such as trucks or railroad cars. In such vehicle loading applications, it is desirable that the transport vehicles are loaded to, but not over, their maximum rated legal capacity. Underloading causes inefficiency in the material hauling cycle and underutilization of the transport vehicles. Overloading causes additional maintenance cost and extra wear on the truck tires and suspension system. Furthermore, the overloaded material may need to be unloaded to decrease load weight, causing additional expense.
Payload measurement is also desirable as a measure of the material hauling operation productivity. The ability to accumulate the weight of the material loaded during a single shift, during a twenty-four hour period, or during any other time period is valuable to an operations manager.
A number of payload measuring devices have been developed. One such device is described in U.S. Pat. No. 4,230,196, issued to Snead on Oct. 28, 1980. Snead discloses a load weighing and accumulating system for a loader. Snead's device includes a pressure sensor for detecting the hydraulic pressure in the lift cylinder of the loader, and a lift arm alignment member which provides a visual aid to the operator to facilitate positioning the lift arm at a predetermined weighing location. To measure the load in the loader bucket, the operator positions the lift arm by visually aligning the alignment members on the lift arm and the vehicle frame. Following a time delay, the load is weighed.
This device has several disadvantages. The amount of data sampled in one-half second is small and results in inaccurate measurements. This is especially significant when the vehicle is operating over rough terrain. The lift cylinder pressure rises when the vehicle hits a bump, and drops when the vehicle experiences "free fall" as it clears the bump. The same sharp pressure deviations occur when the vehicle encounters a surface depression, except the pressure first decreases then increases.
In a typical loader operation environment, the ground surface is uneven. The Snead device requires that the vehicle dynamics be at a steady state when the data sampling is taking place, which is not possible when the vehicle is traveling. Weighing accuracy is severely compromised because of the small data sampling window. The Snead device also requires that the operator maintain the position of the lift arm while measurement is taking place. This requirement has two implications. Because the cylinder pressure can vary over a large range depending on the lift arm position, if the operator fails to align the lift arm, the resultant payload measurement may be grossly erroneous. In addition, requiring the operator to stop the lift arm and position it with the alignment members interrupts the operator's pace and disrupts the loading process.
Another device, disclosed in U.S. Pat. No. 4,055,255, issued to Vasquez on Oct. 25, 1977, describes an apparatus and method for carrying and delivering payload of a predetermined weight. The Vasquez device senses hydraulic pressure in the cylinder supporting the bucket. Because of the peculiar configuration of the vehicle implement linkage, the lift cylinder of the bucket is generally upright except when the bucket is lowered to obtain a load. Therefore, the Vasquez device is a special application payload weighing device not suitable for usage on loaders having different and more conventional linkage configurations.
In many applications, the loading operation is performed while the vehicle is on a ramp or other steep incline. If the vehicle is tilted in the fore-aft direction or the side-to-side direction, the force exerted on the lift cylinder by the implement linkage is different from the force exerted when the vehicle is operating on a level surface. For example, if the linkage geometry is in a position in which the center of gravity of the bucket and linkage is above the elevation of the linkage connection pivot pin and the front of the vehicle is raised to an elevation substantially above the elevation of the rear of the vehicle, the linkage connection pivot pin bears a greater proportion of the load than it does when the vehicle is level. Therefore, the force exerted on the lift cylinder decreases. In the extreme, when the line of action of the force of gravity on the center of gravity of the bucket and linkage extends through the center line of the linkage connection pivot pin, there will be a minimum of force exerted on the lift cylinder. Likewise, if the vehicle is tilted to the side, the linkage connection pivot pin bears a side load and, consequently, less force is exerted on the lift cylinder by the lift linkage. While operating on a slope, all of the above noted payload monitors will indicate an erroneous payload.
The present invention is directed to overcoming one or more of the problems set forth above.